Filter wheel for portable infrared detector



JJU"'LOL "j, Ql-hbn UUM OR Jan 11 1966 H. R. cARLoN ETAI. 3,228,292

FILTER WHEEL FOR PORTABLE INFRARED DETECTOR med March 1e. 1961 United States Patent O 3,228,292 FILTER WHEEL FOR PORTABLE INFRARED DETECTOR Hugh R. Carlon, Edgewood, and Harvey Tannenbaum, Baltimore, Md., assignors to the United States of America as represented by the Secretary of the Army Filed Mar. 16, 1961, Ser. No. 96,337 3 Claims. (Cl. 88-111) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to us of any royalty thereon.

This invention relates to a filter medium for use in infra-red detection devices and to a device which uses said medium. It is well known that practically all the major countries in the world have in their arsenals supplies of the so-called nerve gases such as the G-agents. The identity of the G-agents is disclosed in the patent to Kramer et al. 2,926,072 (col. l, lines 41-47). These nerve gases cannot be detected by the usual senses since they are odorless, tasteless, and colorless. Several schemes for their detection have been devised. The colorirnetric system is exemplified by the above mentioned Kramer patent. An infra-red detection system is shown by Carpenter et al. in Patent 2,930,893.

The instant device is a distinct improvement in the device disclosed by Carpenter et al. A particular object of the present invention is to provide a portable selfcontained infra-red gas or aerosol detector with only one moving part.

A further object of this invention is to provide an inexpensive device that is readily adaptable to the detection of a large number of atmospheric contaminants.

A still further object of this invention is to provide a highly sensitive instrument which provides in a matter of seconds a visual and aural alarm in the presence of a nerve gas.

A final object of this invention is to provide a new type wnbandje/tlmhsie!ifeai/range that is inexpensive and rugged. These and other objects and advantages of the invention will become apparent as the description proceeds.

In the drawings:

FIG. 1 is a schematic drawing of the entire device showing the relationship of the optical and electronic components.

FIG. 2 shows the transmission spectra of the polyethylene film with and without talc (hydrated magnesium silicate) powder applied to the surface of the film.

FIG. 3 shows the details of the rotating filter wheel shown schematically in FIG. 1.

FIG. 4 is a cross sectional view taken on line 4 4 of FIG. 3.

The complete device as is illustrated by FIG. 1 can be housed in one container. In such cases the path of the infra-red radiation is relatively short and the container must be provided with vent holes so that a sample of the contaminated atmosphere will pass between the infra-red source and the bolometer detector.

The infra-red rays emanate from an infra-red source which may be a Nemst glower, heated wire, or heated cavity. They are reflected by a spherical mirror which concentrates the available energy on the active fiake of the thermistor bolometer. In order to exclude all radiation below about 7.2 microns in wave length an indium antimonide optical filter is placed adjacent to the bolometer between the infra-red source and the bolometer.

The rotating filter wheel which is shown in FIG. 1 and detailed in FIGS. 3 and 4 will now be described. This wheel consists of a center disk 3 and an outer mounting ring 9 which may be made of any available rigid plastic such as Plexiglas or Lucite. The plastic film 5 is a circular piece of polyethylene film to which has been added a semi-circular piece or sector 7 of polytrifiuoromonochloroethylene film which for convenience is called fiuorothene film (Kel-F). The part of the polyethylene disk 5 not covered by the polytriuoromonochloroethylene is impregnated with a mineral powder such as talc (hydrated magnesium silicate) by a method which will be described. There need be no polyethylene substrate under the fiuorothene film, if so desired.

The infra-red light after passing through the optical filter and the filter wheel which is rotating at 1800 r.p.m. falls upon the active ake of the thermistor bolometer. This active ake is in series with the reference ake in the illustrated electronic circuit. The filter wheel is so designed that the fiuorothene sector absorbs infra-red radiation in the 8.5 to 9.1 microns and 10.1 to 10.4 micron ranges while transmitting at 9.8 microns. Radiation through the other wheel sector is attenuated at 9.8 microns, due to the impregnated talc. The filter wheel is made to balance optically so that the two sectors transmit equal amounts of energy, although of different wave lengths, and no signal is produced in the detector circuit as the wheel rotates. If a nerve gas is present, some 9.8 micron radiation normally transmitted by the uorothene wheel sector is absorbed by the gas so that the radiation striking the active flake is then fluctuating at the rate of rotation of the filter wheel, and an alternating current signal is produced. When this condition exists, the weak alternating current signal set up in the bolometer circuit is preamplified and then amplified so that it may operate the alarm, after rectification of the signal to direct current is accomplished. This may be achieved by using synchronous switching as a noise rejection precaution.

The alarm used is a matter of choice since the power from the amplifier is adequate to actuate a relay which in turn can operate a lamp or a buzzer, not shown in the drawing. Alternatively, the alarm may consist of a (micro) ammeter which is calibrated to give the concentration of the contaminant.

The filter wheel is prepared as follows. A circular disk of 2 mil polyethylene is cut from the stock and one half of it is covered-with masking tape, front and rear, if it is desired to use a polyethylene substrate under the fiuorothene. Otherwise, a semi-circle of polyethylene would be used. Reagent grade talcum powder is spread over the exposed surface of the disc and rolled into it with a small machined-brass roller, although rolling is not strictly necessary, since scrubbing the film with a coarse paper is sometimes adequate. The excess talc is redistributed, re-rolled, the process being repeated until the desired amount of talc is imbedded in the surface of the plastic film. The remaining talc is then wiped from the surface with a fairly coarse paper wiping tissue.

The wiping is repeated several times, with considerable pressure applied, until all talc which is not imbedded is removed. This process produces a surface uniformly impregnated with very fine talc particles generally less than 3 microns in diameter. Since the sharpness of the talc absorption band at 9.8 microns is dependent on the fineness of the particles, a very sharp band results which is definitely superior to any which may be obtained through a process wherein said form of a binder such as wax is used to hold the talc particles on the substrate. In addition, the particles become so firmly imbedded in the plastic substrate that no known method short of destruction of the film itself, will dislodge them. The film maybe fiexed, folded, doubled, rubbed, immersed in liquids (which are not solvents for polyethylene), and even crumpled in the hand without damage to the talc surface. Furthermore, no protective covering is used or needed for this filter.

In the process just described, if 2 mil polyethylene is saturated with talc particles, an absorption of 80-85% will be effected at 9.8 microns, with absorptions of only about 20% at wavelengths of 8.5 and 10.8 microns. In this particular alarm application it was desired to `increase the 9.8 micron absorption and to generally attenuate the talc wheel sector transmission so as to bring the transmission of both halves of the filter wheel to a condition of approximately equal magnitude and thus optically balance the filter wheel. This was accomplished -by saturating the other face of the exposed semi-circle or sector and placing over it an additional se-mi-circle of 4 mil polyethylene that is likewise saturated front and back with talc. Combinations of various thickness of polyethylene used in this manner can yield filter wheels with almost any desired transmission characteristics for the talc sector.

The remaining half of the wheel (if the full polyethylene substrate was used) is unmasked and covered with a piece of half-mil uorothene (poly-triliuoromonochloroethylene). The fiuorothene may be held in place temporarily with ltape as the mounting ring is prepared. The mounting ring is machined to the outside diameter of the wheel, from Lucite, consisting essentially of polymerized methacrylate esters. Since the wheel will be rotated in the instrument, at 1800 r.p.m., Lucite is chosen because it provides suicient rigidity with low mass, as a large mass of the circumference of the wheel cannot be tolerated at this 4rotational speed. However, the mass is sufiicient to hold the plastic film fiat as the wheel rotates. The mounting ring disc is cut out in the center to form a ring having a width, at a circumference, of approximate-ly 5 mm. The completed wheel is laid over the ring and attached to it with household cement such as that sold by the E. I. du Pont de Nemours Company under the tradename-Duco. Since the cement will not adhere permanently to the plastic, a series of circumferential holes are made through .the plastic with a hot miniature soldering iron. The cement is then applied through the holes and onto the Lucite ring, forming small rivets of cement which hold the film in place. A second similar mounting ring is then placed over the wheel, providing added rigidity. The wheel is completed through the addition of two small Lucite discs, approximately 2 cm. in diameter, to the wheel. These discs are then centerpunched so that the wheel may be mounted. Additional support is obtained for the plastic disc in this manner.

Obviously this filter-producing process can be extended lto the use of other powdered minerals, as has been demonstrated for silica gel, calcium carbonate, potassium period-ate, and various nitrates and sulfates, to name a few. Each of these minerals has a specific absorption in the infra-red band as is set forth in the article by Hunt et al. in Analytical Chemistry 22:1479-97 (1950).

By this means, one can extend the instant invention to detect other gases or vapors that have an absorption in the infra-red comparable to the minerals used. The filter wheel does not lhave rto be made of two sectors by any number of alternating sectors may be used so long as the wheel is optically balanced (in the infra-red band) when it is rotated. Obviously, the use of a greater number of sectors increases the frequency of the alternating current generated in `the bolometer circuit. The actual revolutions per minute of the direct current motor and the filter wheel can thus be reduced if it is so desired.

The optical path is not restricted to the straight line path shown in FIG. 1 but can be made to take a folded path by means of plane mirrors in order to take up less space.

When using an optical path of twenty inches, the instrument is capable of detecting as little as 2 micrograms of nerve gas per liter of air in a matter of seconds i.e. no more than ten seconds.

Instruments utilizing the identical principle described above could be constructed to have stationary filters with rotating optics or any of several related designs.

The 2 microgram per liter sensitivity noted is by no means near the theoretical limit for such a system.

We claim:

1. An med hanclapserntnnfilter having a strong absorption band inthe range 9.2 to 10.4 microns and sub- Stantionally no other absorption in the range 7-13 microns comprising a thin film of polyethylene which has impregnated therein in a film surface thereof, finely ground talc of a particle size generally less than 3 microns in diameter.

2. A filter wheel useful as a rotating light chopper comprising an outer mounting ring, a center disc, a circular sheet of polyethylene fil-m, half of which is impregnated with a fine mineral powder selected from the group consisting of talc, silica gel, calcium carbonate, and potassium periodate, and a semi-circular sheet of polytrfluoromonochloroethylene mounted over the untreated portion of the circular sheet.

3. A filter wheel comprising an outer mounting ring, a center disk, a plurality of sectors of polyethylene film impregnated with a fine mineral powder selected from the group consisting of talc, silica gel, calcium carbonate and potassium periodate, and a plurality of sectors of polyethylene film covered with a film of polytrifluoromonochloroethylene, said impregnated and covered sectors being equal in number and alternating around the surface of said wheel.

References Cited by the Examiner UNITED STATES PATENTS 2,668,478 2/1954 Scroder 88-112 2,685,649 8/1954 Miller Z50-43.5 2,689,801 9/1954 DAlelio 117-9 2,708,389 5/1955 Kavanagh 88-112 2,775,160 12/1956 Foskett et al. 8814 2,930,893 3/1960 Carpenter Z50-43.5 2,952,575 9/ 1960 Baltzer.

2,953,469 9/1960 Fox 117-9 2,992,938 7/1961 McCarville et al.

3,091,690 5/1963 McHenry Z50-43.5

OTHER REFERENCES Henry: The Transmission of Powder Films in the Infra-Red, Article in Journal of the Optical Society of America, vol. 38, September 1948, pages 775-789 cited.

Hunt: Infrared Absorption Spectra of Minerals, Article in Analytical Chemistry, vol. 22, December 1950, pages 1478-1497 cited.

Pfund: Optical Properties of Metallic and Crystalline Powders, Article in Journal of the Optical Society 0f America, vol. 23, October 1933, pages 375-378 cited.

Smith: Use of Polytriuorochloroethylene in Infra- Red and Raman Spectroscopy, Article in Journal of the Optical Society of America, vol. 39, September 1949, pages 780 and 781 cited.

DAVID H. RUBIN, Primary Examiner. ARTHUR GAUSS, Examiner. JAMES W. LAWRENCE, Assistant Examiner. 

1. AN INFRA-RED BAND ABSORPTION FILTER HAVING A STRONG ABSORPTION BAND IN THE RANGE 9.2 TO 10.4 MICRONS AND SUBSTANTIALLY NO OTHER ABSORPTION IN THE RANGE 7-13 MICRONS COMPRISING A THIN FILM OF POLYETHYLENE WHICH HAS IMPREGNATED THEREIN IN A FILM SURFACE THEREOF, FINELY GROUD TALC OF A PARTICLE SIZE GENERALLY LESS THAN 3 MICRONS IN DIAMETER. 